The present invention relates generally to implantable medical prostheses, more specifically, implantable pulse generator for treating or controlling neurological and neuropsychiatric disorders using pulsed vagal nerve stimulation.
The apparatus and methods disclosed herein also may be appropriate for the treatment of other conditions, as disclosed in co-pending application filed on Aug. 29, 2001, entitled APPARATUS AND METHOD FOR TREATMENT OF UROLOGICAL DISORDERS USING PROGRAMMERLESS IMPLANTABLE PULSE GENERATOR SYSTEM.
Neuromodulation of cranial nerve using pulsed electrical stimulation has utility as an adjunct (add-on) therapy for neurological and neuropsychiatric disorders such as epilepsy, severe depression, dementia including Alzheimer""s disease, compulsive eating disorders, sleeping disorder, coma, diabetes, neurogenic/psychogenic pain etc. This patent is directed to a system of implantable lead and pulse generator which is programmerless, and the pulse generator being controlled by only an external magnet.
Implanted pulse generator (IPG) for neuromodulation systems generally consist of an implantable lead, an implantable pulse generator, and an external programmer for non-invasively programming the parameters of the IPG. One such prior art is shown in FIG. 1.
The programmer generally is a microprocessor-based device, which provides a series of encoded signals to the implanted pulse generator by means of a programming head which transmits radio-frequency (RF) encoded signals to pulse generator according to the telemetry system laid out in that system. Such a system requires an antenna which is connected to input/output circuit for purposes of uplink/downlink telemetry through an RF telemetry circuit.
A built-in antenna enables communication between the implanted pulse generator and the external electronics (including both programming and monitoring devices) to permit the device to receive programming signals for parameter changes, and to transmit telemetry information, from and to the programming wand. Once the system is programmed, it operates continuously at the programmed settings until they are reprogrammed (by the attending physician) by means of the external computer and the programming wand.
In such a system any programming methodology may be employed so long as the desired information can be conveyed between the pulse generator and the external programmer.
Generally, implanted pulse generators work quite well, except their manufacturing costs and corresponding selling price tends to be high and places a burden on the health care system. A significant part of the cost is attributed to the programmability of the implanted device, as well as, the computer-based programmer itself.
Historically, implantable neurostimulator technology evolved based significantly on the existing cardiac pacemaker technology. Both are essentially electrical pulse generators. However, there is one significant difference, which is, for a cardiac pacemaker to function properly it needs to sense the electrical activity of the stimulating tissue. Therefore, in a cardiac pacemaker an external programmer is an integral part of the system to program the sensitivity. A system for nerve modulation is not dependent upon sensing from the stimulating tissue such as the nerve, before providing electric pulse stimulation.
Thus, by incorporating a limited number of predetermined/prepackaged programs into the implantable pulse generator, a significant manufacturing and development cost reduction for the system can be achieved, with very little loss of functionality.
One of the fundamental features of the nervous system is its ability to generate and conduct electrical impulses. These can take the form of action potentials, which is defined as a single electrical impulse passing down an axon, and is shown schematically in FIG. 2. The top portion of the figure (A) shows conduction over mylinated axon (fiber) and the bottom portion (B) shows conduction over nonmylinated axon (fiber). These electrical signals will travel along the nerve fibers.
The nerve impulse (or action potential) is an xe2x80x9call or nothingxe2x80x9d phenomenon. That is to say, once the threshold stimulus intensity is reached an action potential 7 will be generated. This is shown schematically in FIG. 3. The bottom portion of the figure shows a train of action potentials.
Most nerves in the human body are composed of thousands of fibers of different sizes. This is shown schematically in FIG. 4. The different sizes of nerve fibers, which carry signals to and from the brain, are designated by groups A, B, and C. The vagus nerve, for example, may have approximately 100,000 fibers of the three different types, each carrying signals. Each axon or fiber of that nerve conducts only in one direction, in normal circumstances.
In a cross section of peripheral nerve it is seen that the diameter of individual fibers vary substantially. The largest nerve fibers are approximately 20 xcexcm in diameter and are heavily myelinated (i.e., have a myelin sheath, constituting a substance largely composed of fat), whereas the smallest nerve fibers are less than 1 xcexcm in diameter and are unmyelinated. As shown in FIG. 5, when the distal part of a nerve is electrically stimulated, a compound action potential is recorded by an electrode located more proximally. A compound action potential contains several peaks or waves of activity that represent the summated response of multiple fibers having similar conduction velocities. The waves in a compound action potential represent different types of nerve fibers that are classified into corresponding functional categories as shown in the table below,
The diameters of group A and group B fibers include the thickness of the myelin sheaths. Group A is further subdivided into alpha, beta, gamma, and delta fibers in decreasing order of size. There is some overlapping of the diameters of the A, B, and C groups because physiological properties, especially in the form of the action potential, are taken into consideration when defining the groups. The smallest fibers (group C) are unmyelinated and have the slowest conduction rate, whereas the myelinated fibers of group B and group A exhibit rates of conduction that progressively increase with diameter.
Compared to unmyelinated fibers, myelinated fibers are typically larger, conduct faster, have very low stimulation thresholds, and exhibit a particular strength-duration curve or respond to a specific pulse width versus amplitude for stimulation. The A and B fibers can be stimulated with relatively narrow pulse widths, from 50 to 200 microseconds (xcexcs), for example. The A fiber conducts slightly faster than the B fiber and has a slightly lower threshold. The C fibers are very small, conduct electrical signals very slowly, and have high stimulation thresholds typically requiring a wider pulse width (300-1,000 xcexcs) and a higher amplitude for activation. Because of their very slow conduction, C fibers would not be highly responsive to rapid stimulation. Selective stimulation of only A and B fibers is readily accomplished. The requirement of a larger and wider pulse to stimulate the C fibers, however, makes selective stimulation of only C fibers, to the exclusion of the A and B fibers, virtually unachievable inasmuch as the large signal will tend to activate the A and B fibers to some extent as well.
The vagus nerve is composed of somatic and visceral afferents and efferents. Usually, nerve stimulation activates signals in both directions (bi-directionally). It is possible however, through the use of special electrodes and waveforms, to selectively stimulate a nerve in one direction only (unidirectionally). The vast majority of vagus nerve fibers are C fibers, and a majority are visceral afferents having cell bodies lying in masses or ganglia in the skull. The central projections terminate largely in the nucleus of the solitary tract, which sends fibers to various regions of the brain (e.g., the thalamus, hypothalamus and amygdala).
Vagus nerve stimulation is a means of directly affecting central function. As shown in FIG. 6, cranial nerves have both afferent pathway 19 (inward conducting nerve fibers which convey impulses toward the brain) and efferent pathway 21 (outward conducting nerve fibers which convey impulses to an effector). The vagus nerve 54 is composed of 80% afferent sensory fibers carrying information to the brain from the head, neck, thorax, and abdomen. The sensory afferent cell bodies of the vagus reside in the nodose ganglion and relay information to the nucleus tractus solitarius (NTS).
FIG. 7 shows the nerve fibers traveling through the spinothalamic tract to the brain. The afferent fibers project primarily to the nucleus of the solitary tract (shown schematically in FIG. 8) which extends throughout the length of the medulla oblongata. A small number of fibers pass directly to the spinal trigeminal nucleus and the reticular formation. As shown in FIG. 8, the nucleus of the solitary tract has widespread projection to cerebral cortex, basal forebrain, thalamus, hypothalamus, amygdala, hippocampus, dorsal raphe, and cerebellum. In summery, it is these projections of the solitary track nucleus to the reticular system and other higher centers in the brain that is responsible for the therapy effects for neurological and neuropsychiatric disorders.
U.S. Pat. Nos. 4,702,254 and 4,867,164 (Zabara) are directed to neurocybernetic prosthesis, where an implantable pulse generator is controlled by an external programmer. The ""254, and ""164 patents on neurocybernetic prosthesis (NCP) point away from the present patent application since NCP utilizes neurocybernetic spectral discrimination by tuning the external current of the NCP generator to the electrochemical properties of a specific group of inhibitory nerves that affect the reticular system of the brain. According to the patent, the spectral discrimination analysis dictates that certain electrical parameters of the NCP pulse generator be selected based on the electrochemical properties of the nerves desired to be activated.
U.S. Pat. No. 4,884,575 (Sanders) is directed to a cardiac pacemaker adapted to generate a first pacing rate, and to selectively increase the rate to higher exercise rate which can be triggered with a time delay. In the Sanders patent, their is no suggestion to have a limited number of prepackaged/predetermined programs built into the pacemaker, and to selectively activate them with only a magnet. In a cardiac pacemaker, an external programmer is essential to adjust the sensitivity of the pacemaker, such that the pacemaker does not compete with the intrinsic rhythm of the heart.
In contrast, in the current patent application for neuromodulation of the vagus nerve in controlling neurological and neuropsychiatric disorders, there is no sensing involved from the stimulation tissue, i.e. the vagus nerve. Therefore, all of the stimulation programs containing the different electrical stimulation parameters, can be built-in, and which can be selectively activated with a magnet. This eliminates the need for an external programmer.
U.S. Pat. No. 5,304,206 (Baker et al) is directed to techniques and apparatus for activating implanted neurostimulators. In the Baker patent, as shown in FIG. 1, the implanted device communicates with a programmer and/or monitor external to the patient""s body by means of asynchronous serial communication, to control and indicate device states. Further, the patient can adjust the implanted generator by finger tapping, whereby the piezoelectric sensor is activated. There is no suggestion in the Baker patent to simplify the implant by having prepackaged/predetermined programs in the implant, and eliminating the programmer.
U.S. Pat. No. 6,205,359B1 (Boveja) is directed to neuromoduation of a cranial nerve such as the vagus nerve for controlling neruologic disorders. The predetermined programs in the ""359 patent, can be activated by manually pressing a button, since the pulse generator is external to the body.
A drawback of the prior art neuromodulation system is that it adds significant cost to the system. In the system of the current invention, high value is provided by eliminating the development of a computer based programmer to control the implanted pulse generator.
Accordingly, an apparatus and method of this invention comprises an implantable pulse generator and lead system which is adapted to provide pulsed electrical stimulation to a cranial nerve. The pulse generator comprises a limited number of predetermined/prepackaged programs built-in and means for accessing the programs with a magnet. The pulse generator also contains means to access the predetermined programs by a magnet. Thereby eliminating the need for an expensive computer based external programmer.